Compressors are widely used worldwide in industrial applications to compress gases. Compressor surge is a phenomenon which occurs in compressors at low volumetric flow rates, and hence limits the minimum capacity of a given compressor. In the operation of a compressor, as the system resistance is increased, the head generated by the compressor increases to overcome this resistance. As system pressure increases, less flow can pass through the compressor, and this will continue up to the maximum head capacity of the compressor. In a characteristic head (H) vs. flow (Q) curve, or head/flow for a given compressor, there can thus be defined a limit in the minimum flow region known as "Surge Flow" or "Surge Point." The head increases with decreasing flow defining a basic slope. Below the surge limit the compressor head/flow curve takes a reverse slope. At this condition, the system back pressure exceeds that at which the compressor is capable of delivering, this causing a momentary backflow condition. At this point in time however system resistance decreases, this causing the back pressure to drop, enabling the compressor to deliver at higher flow than when the surge began. If the opposition to flow downstream of the compressor is unchanged, operation again follows back along the peak head/flow curve until peak head delivery is again readied, this producing the cyclic action known as surge. Thus, during surge, the pressure developed by the compressor can drop momentarily and reach a level where flow actually reverses direction. Although surge energy varies between one compressor and another, considerable damage can be done to a compressor if it is operated in this regime due to vibration, noise, shaft axial movement and overheating which can produce mechanical damage. Ideally however, during periods of reduced flow rates, it is desirable to operate a compressor on the head/flow curve above but as nearly as possible to the surge limit to maximize compressor efficiency, minimize recycle, and thus reduce operating costs.
In operation, to prevent a compressor from operating in surge, some form of control or anti-surge protection device is required. Typically, e.g., the surge control device can be a low flow vent, or a recycle line set to open at some flow higher than surge to return compressed gas from the discharge side to the suction side of the compressor, the device being set to provide a safety margin 10-20% above surge flow to protect against errors in calculated flow rates due to variations caused by changing gas conditions, e.g., gas molecular weight, pressure, temperature, etc., on the head/flow curve at any given operating speed. This safety margin is required to protect the compressor from surge flow, but the higher the set point above the point of surge flow on the head/flow curve the more recycle or venting is required, this resulting in higher operating costs.
Various means have been employed to monitor various operational parameters of a compressor, and to use these measurements to control the operation of the compressor to avoid surge. On one of the more recent control systems, as described in U.S. Pat. No. 4,156,578 which issued on 05/29/79 to Joram Agar et al, surge is avoided by the measurement across the inlet and discharge side of a compressor of such variables as compressor inlet pressure P.sub.1, compressor outlet pressure P.sub.2, and the differential pressure p across a throttling member disposed in an inlet duct of the compressor. The values P.sub.1 and p are input to a first divider to produce a signal proportional to p/P.sub.1 which is input to a computer. The values P.sub.1 and P.sub.2 are input to a second divider to produce a signal P.sub.2 /P.sub.1 which is also input to the computer. The computer compares the values of P.sub.2 /P.sub.1 and p/P.sub.1 with preprogrammed information based on parameters whose values depend on the characteristics of the compressor (1) operated at conditions which avoids surging and (2) operated at conditions which will produce surging. When the computer output signal, operated at conditions which avoids surging, is output to a two mode controller the latter will maintain the valve of a by-pass line from the discharge side to the suction side of the compressor closed. Conversely, when the computer output signal to the two mode controller indicates conditions which will produce surge the valve of the by-pass line is opened to pass additional gas to the suction side of the compressor to avoid surging.
In U.S. Pat. No. 4,230,437, which was issued to Robert M. Bellinger et al on 10/28/80, there is also described a compressor surge control for a compressor system wherein gas is recycled via a line from the discharge side to the suction side of a compressor. In this system, the amount of recycled gas is minimized by using a first controller to provide a floating set point for a second controller.
Such antisurge systems, and others have advanced the state of the art, but further improvements are nonetheless needed.